Implication of pili as virulence factors for gonococci (Gc) in causation of acute gonorrhea has led to interest in genetic control of pilus expression and of subunit variation among piliated Gc derived from a single strain. "On-off" control of piliation has been examined by blot (Southern) hybridization of endonuclease-cleaved Gc genomic DNA with probes containing pilin-encoding information in DNA that was molecularly cloned from pilus+Gc. Evidence for two different genetic mechanisms relating to pilus+-- pilus- phenotype changes is the present interpretation of the findings: some, but not all, pilus+ greater than pilus transitions (strain MS11) coincide with recombinational events in Gc genomic DNA; only those pilus+ to pilus changes that accompany no discernible change in blot hybridization patterns yield pilus- progeny that can subsequently undergo pilus- greater than pilus+ transitions. These data are interpreted as reflecting (1) recombination/deletion events in pilus structural gene DNA, and (2) genetic control based on nonpilin-structural-gene portion(s) of the Gc pilin/pilus operon. These studies are also aimed at defining the genetic mechanism(s) involved in the high frequency changes in pilin subunits which are found among variants of a given strain and which accompany changes in both antigenicities and host cell-adherence properties of pili.